1. Field of the Invention
This invention relates to the mounting of flexible printing plates and more particularly to pressure sensitive adhesive sheet materials for mounting such printing plates.
2. Description of the Prior Art
"Flexographic" printing involves a flexible printing plate mounted to the drum of a printing press. "Drum" as used herein, refers to removable sleeves, cylinders and the like upon which the printing plate is mounted. The flexible printing plates are formed of molded compressible rubber with the particular design to be printed being defined by the highest portions of the printing plate.
The dimensional manufacturing tolerances for the flexographic pringting plates must be closely controlled in order to produce printed images free of low spots and other imperfections. In operation, the printing plate is mounted to the press drum and the press rolls are adjusted to provide the proper pressure for ink transfer to the plate, and ink transfer from the plate to the substrate to be printed. It is desirable to make as few adjustments as possible throughout a printing run.
A critical link in the flexographic printing apparatus is the adhesion of the printing plate to the press drum. The adhesive material used should be easily handled, should not affect the printing process and should be easily removed.
One technique for adhering the printing plate to the press drum is by applying a liquid or semi-solid adhesive to the drum or plate and placing the drum and plate in mating engagement. After use, the adhesive must be peeled from the plate and/or drum in pieces because the cohesion of the adhesive is less than the adhesion of the adhesive to the printing plate and/or the drum. Removal of the adhesive is a time-consuming process.
It has, therefore, been proposed to use pressure sensitive adhesive coated sheet materials which are easily applied to both the printing plate and the press drum. The sheet materials have been fabricated from vinyl sheet coated with a pressure sensitive adhesive on both sides. Although the vinyl has adequate integrity to be removed from the drum and/or plate and remain integral, the vinyl does not retain the same gauge throughout a long print run because of lack of memory, and undesirable adjustments must be made to the press over the course of printing to provide a quality printed product.
Another proposed pressure sensitive sheet material has been flexible foam coated on both sides with a pressure sensitive adhesive. Although the foam has desirable properties with respect to memory, it does not have sufficient integrity to remain as an integral sheet when it is delaminated from the printing plate and/or press drum, and thus has deficiencies similar to liquid or semi-solid adhesives applied directly to the drum.
A further problem associated with the adherence of flexible printing plates to printing drums is that the particular adhesive selected must have adhesive properties to various rubber or plastic printing plates and to press drums of various metal compositions. In some instances, it has been proposed to use a sheet material which has different adhesives coated on opposing sides in order to provide adhesion to the plate on one side and the press drum on the opposing side.
In accordance with the present invention, a sheet material for mounting flexible printing plates to the drum of a printing press is provided which has a uniform gauge and maintains this gauge during the printing process. Further, the sheet material in accordance with the invention preferably uses the same adhesive on both sides of the sheet material.
In addition, the sheet material of the present invention has sufficient integrity so that it remains an integral sheet upon removal from the drum and press.